What is iowait and how does it affect Linux performance?

I/O wait or iowait, wait, wa, %iowait, or wait% is often displayed by command-line Linux system monitoring tools such as top, sar, atop, and others. On its own, it’s one of many performance stats that provide us with an insight into Linux system performance.

I/O wait came up in a recent discussion with a new client of mine. During our support call, they reported load spikes of 60 to 80 on their 32 CPU core system. This resulted in slow page loading, timeouts, and intermittent outages. The cause? Storage I/O bottleneck as hinted at first by a consistently high iowait, and then later confirmed with additional investigation.

What is I/O wait? How does I/O wait affect Linux server performance? How can we monitor and reduce I/O wait related issues? Continue reading for the answers to these questions.

iostat iowait example

 

What is I/O wait?

I/O wait applies to Unix and all Unix-based systems, including macOS, FreeBSD, Solaris, and yes Linux.

I/O wait (iowait) is the percentage of time that the CPU (or CPUs) were idle during which the system had pending disk I/O requests. (Source: man sar) The top man page gives this simple explanation: “I/O wait = time waiting for I/O completion.” In other words, the presence of I/O wait tells us that the system is idle at a time when it could be processing outstanding requests.

“iowait shows the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request.” –  iostat man page

When using Linux top and other tools, you’ll notice that a CPU (and its cores) operate in the following states: us (user), sy (system), id (idle), ni (nice), si (software interrupts), hi (hardware interrupts), st (steal) and wa (wait). Of these, the user, system, idle, and wait values should add up to 100%. Note that “idle” and “wait” are not the same. “Idle” CPU means there is no workload present while, on the other hand, “wait” (iowait) indicates when the CPU is waiting in an idle state for outstanding requests.

If the CPU is idle, the kernel will ascertain if there are any pending I/O requests (i.e., SSD or NFS) originating from the CPU. If there are, then the ‘iowait’ counter is incremented. If there’s nothing pending, then the ‘idle’ counter is incremented instead.

 

I/O wait and Linux server performance

It’s important to note that iowait can, at times, indicate a bottleneck in throughput, while at other times, iowait may be completely meaningless. It’s possible to have a healthy system with high iowait, but also possible to have a bottlenecked system without iowait.

I/O wait is simply one of the indicated states of your CPU / CPU cores. As such, a high iowait means your CPU is waiting on requests, but you’ll need to investigate further to confirm the source and effect.

For example, server storage (SSD, NVMe, NFS, etc.) is almost always slower than CPU performance. Because of this, I/O wait may be misleading, especially when it comes to random read/write workloads. This is because iowait only measures CPU performance, not storage I/O.

Although iowait indicates that the CPU can handle more workload, depending on your server’s workload and the way that load performs computations or makes use of storage I/O, it isn’t always possible to solve I/O wait. Or, not feasible to achieve a near-zero value.

You will have to decide based on end-user experience, database query health, transaction throughput, and overall application health, whether or not the iowait reported, indicates poor Linux system performance, or not.

For example, if you see low iowait of 1 to 4 percent, and you then upgrade the CPU to 2x the performance, the iowait will also increase. A 2x faster CPU with the same storage performance = ~ 2x the wait. You’ll want to consider your workload to determine which hardware you should pay attention to first.

 

Monitoring and reducing I/O wait related issues

iostat -xm 2 (check for iowait)

Let’s look at some useful tools we can use to monitor I/O wait on Linux.

  • atop – run it with -d option or press d to toggle the disk stats view.
  • iostat – try it with the -xm 2 options for extended statistics, in megabytes, and in two-second intervals.
  • iotop – top-like I/O monitor. Try it with the -oPa options to show the accumulated I/O of active processes only.
  • ps – use auxf, then under the “STAT” column “D” usually indicates disk iowait.
  • strace – view the actual operations issued by a process. Read the strace man page.
  • lsof – after you’ve identified the process responsible, use -p [PID] to find the specific files.

Reducing I/O wait related issues

Take the following steps to reduce I/O wait related issues.

  • Optimize your application’s code and database queries. This can go a long way in reducing the frequency of disk reads/writes. This should be your first approach because the more efficient your application is, the less you’ll have to spend on hardware longterm. See also: 100 Application Performance Monitoring (APM) & Observability Solutions.
  • Keep your Linux system and software versions up-to-date. Not only is this better for security, but more often than not, the latest supported versions offer notable performance improvements, whether its Nginx, Node.js, PHP, Python, or MySQL.
  • Make sure that you have free memory available. Enough free memory so that around half of the server’s memory is being used for in-memory buffers and cache, rather than swapping and paging to disk. Of course, this ratio will differ case by case. Therefore, be sure that you are not swapping and that kernel cache pressure isn’t high due to a lack of free memory.
  • Tweak your system, storage device(s), and the Linux kernel for increased storage performance and lifespan.
  • Finally, if all else fails: upgrade storage devices to faster SSD, NVMe, or other high throughput storage devices.

 

Conclusion

The iowait statistic is a useful performance stat for monitoring CPU utilization health. It notifies the Sysadmin when CPU is idle and can possibly perform more computations. At which point, we can then use observabilitybenchmarking, and tracing tools such as those listed and linked to above, to put together a full picture of the system’s overall I/O performance. Your main goal should be to eliminate any iowait that’s a direct result of waiting on disk, NFS, or other storage-related I/O.

Tags: , , , , ,